Computer tomography method for a periodically moving object

ABSTRACT

The invention relates to a computer tomography method in which a periodically moving object, in particular an organ of the body, is irradiated by a cone-shaped beam cluster ( 4 ) along a trajectory which runs on a cylindrical surface. The radiation transmitted through the object is measured by means of a detector unit ( 16 ), and at the same time the periodic movement of the object is recorded. In order to reconstruct the absorption distribution of the object, the measured values or the corresponding beams are rebinned to form a number of parallel projections, where for each of these projections a measured value is determined whose beam irradiates the object. The point in time at which this measured value was acquired is allocated to the respective projection. For the reconstruction, which may for example be carried out using a filtered back-projection, only projections whose allocated points in time lie within a predefined, specific time range (H 1 ) within a period of the object movement are used.

The invention relates to a computer tomography method in which aperiodically moving object, in particular an organ of the body, which islocated in an examination area is irradiated by a cone-shaped beamcluster. The invention also relates to a computer tomography scanner forcarrying out the method and to a computer program for controlling thecomputer tomography scanner.

Within the context of this invention, the expression “periodic movement”refers to movements in which a series of object states are repeatedlyassumed by the object, always in the same sequence, during ameasurement. An object state may in this case be defined, for example,by the position of the object in the examination area and the shape ofthe object or by a specific measured signal which is allocated to theobject state. This measured signal may, for example, be the signal of anelectrocardiograph if the moving object is a heart. If anelectrocardiograph supplies the same measured signals at differentpoints in time, then it is assumed that the object states at thesepoints in time are the same. Within the context of the invention,however, the term “periodic movements” also means movements which couldbe referred to as “quasi-periodic” and in which object states that arerepeatedly assumed by the object are not exactly identical but aresubstantially identical. The same applies to the time duration betweentwo substantially identical, repeating object states, which timeduration need only be substantially constant during the measurement inorder to be able to refer to the corresponding movement as “periodic”within the context of this invention. Different period time durationsare “substantially identical” if their time difference is small comparedto the periodic time durations for the respective application.Accordingly, two object states are substantially identical if theirdifference is small compared to the differences which the object passesthrough within an entire period. Therefore, for example, movements ofpulsating technical or biological objects such as organs of the body orveins may be referred to as “quasi-periodic” and hence also as“periodic”. Moreover, the term “periodic movement” also encompassesattenuated oscillations or movements in which the object repeatedlyassumes only some, that is to say not all, states at substantiallyidentical time intervals.

In known methods of the type mentioned above, the spatial profile of theabsorption or of the attenuation of the radiation in the periodicallymoving object is reconstructed from measured values acquired using adetector unit. In this case, the movement of the object leads to themeasured values containing information from different object states, andthis leads to movement artefacts in the reconstructed data record.

Therefore, in known reconstruction methods use is only made of measuredvalues which have been acquired in time ranges in which the object hasmoved relatively little, that is to say in which the object stateschange only slightly. In one of these methods, the beams firstly undergoparallel rebinning in order to reduce the computational complexity. Inparallel rebinning, the cone-shaped beam cluster for each radiationsource position is split on the trajectory into beam fans, the beams ofwhich in each case lie in a plane which runs parallel to the centralaxis of the circular or helix-like trajectory. These beam fans are thendivided into groups such that each group contains only beam fans thatare parallel to one another. The beam fans of a group are referred to asa projection. A point in time is allocated to each projection, saidpoint in time being the same as the point in time at which the measuredvalues corresponding to the central beam fan of the projection wereacquired. It is assumed that all measured values of a projection wereacquired at this point in time.

In the reconstruction of the absorption distribution from the rebinnedbeams or measured values, use is only made of projections which wereacquired during predefinable time ranges in which the movement of theobject is relatively small. Since—apart from in respect of the centralbeam fan—the point in time of a projection does not coincide with theactual acquisition time of a measured value of this projection, for thereconstruction use is frequently made of measured values whose beams doindeed come into contact with the object but were acquired at points intime which lie outside the predefined time ranges. This leads tomovement artefacts which increase as the distance of the object from thecentral beam fan increases.

It is an object of the present invention to specify a method in whichthese movement artefacts are less pronounced. This object is achievedaccording to the invention by a computer tomography method comprisingthe steps

-   -   using a radiation source (S) to generate a cone-shaped beam        cluster (4) which passes through an examination area (13) and a        periodically moving object which is located in the examination        area (13),    -   producing a relative movement between the radiation source (S)        on the one hand and the object located in the examination area        (13) on the other hand, where a trajectory, along which the        radiation source moves relative to the object, runs on an        imaginary cylindrical surface that surrounds the object,    -   using a detector unit (16) to acquire measured values which        depend on the intensity in the beam cluster (4) on the other        side of the object, during the relative movement,    -   recording the periodic movement of the object during the        acquisition,    -   reconstructing a spatial distribution of the absorption of the        periodically moving object from the measured values with the aid        of the recorded periodic movement of the object, comprising the        steps:

-   a) determining the spatial area taken up by the object in the    examination area (13),

-   b) subjecting the measured values to parallel rebinning in order to    form a number of groups, where the beams corresponding to the    measured values of each group form beam fans (41 . . . 45) which lie    in planes that are parallel to one another and to the axis of    rotation,

-   c) determining for each group a measured value whose beam irradiates    the spatial area taken up by the object, and allocating to the    respective group the point in time at which this measured value was    acquired,

-   d) determining those groups whose points in time, allocated in step    c), lie within periodic, predefined time ranges (H₁),

-   e) reconstructing the absorption distribution in the object from the    measured values belonging to the groups determined in step d).

In contrast to known methods, in the invention there is determined foreach group or projection a measured value whose beam irradiates theobject, and the point in time at which this measured value was acquiredis allocated to the projection. This leads to the time intervals betweenthe point in time allocated to a projection and the acquisition times ofsaid measured values of this projection, which irradiate the object,being smaller than in known methods. This reduces the incorrect use ofmeasured values which lie outside the predefined time ranges. Since thetime ranges are generally selected such that they correlate with phasesof the object movement which do not contain much movement, movementartefacts are reduced.

Claim 2 describes one refinement in which the spatial area taken up bythe object in the examination area is determined with a lowcomputational complexity by reconstruction and segmentation of theobject.

In Claim 3, for each group there is determined the measured value whosebeam runs through the geometric center of the object. The point in timeat which this measured value was acquired is allocated to the respectivegroup. This leads to a further reduction in the incorrect use ofmeasured values which lie outside the predefinable time range, and thisfurther reduces the movement artefacts.

Claim 4 shows one refinement in which the periodically moving object isa heart and in which reconstructed images of a high quality can beproduced inter alia with the aid of an electrocardiograph.

In claim 5, the predefinable time ranges are selected such that theobject moves less in these time ranges than in other time ranges, andthis leads to a further reduction of the movement artefacts in thereconstructed images.

Claim 6 defines one refinement in which the reconstruction is carriedout using a filtered back-projection, and this leads to a good imagequality of the reconstructed object while entailing a low computationalcomplexity.

In claim 7 the radiation source moves relative to the examination areaon a helix-like or circular trajectory, and this leads to reconstructionresults with a good image quality.

A computer tomography scanner for carrying out the method is describedin claim 8.

Claim 9 defines a computer program for controlling a computer tomographyscanner as claimed in claim 8.

The invention will be further described with reference to examples ofembodiments shown in the drawings to which, however, the invention isnot restricted.

FIG. 1 shows a computer tomography scanner which can be used to carryout the method according to the invention.

FIG. 2 shows a flowchart of the method according to the invention.

FIG. 3 shows a beam cluster of parallel rebinned beam fans.

FIG. 4 shows a flowchart for a back-projection.

FIG. 5 shows an electrocardiogram.

FIG. 6 shows a flowchart for a further back-projection.

FIG. 7 shows the dependence of an α factor on time.

FIG. 8 shows the dependence of a β factor on time.

The computer tomography scanner shown in FIG. 1 comprises a gantry 1which can rotate about an axis of rotation 14 that runs parallel to thez direction of the coordinate system shown in FIG. 1. For this purpose,the gantry 1 is driven by a motor 2 at a preferably constant butadjustable angular velocity. A radiation source S, for example an X-raygenerator, is attached to the gantry 1. Said radiation source isprovided with a collimator arrangement 3 which masks, from the radiationgenerated by the radiation source S, a cone-shaped beam cluster 4, thatis to say a beam cluster which has, both in the z direction and in adirection perpendicular thereto (i.e. in a plane perpendicular to theaxis of rotation), a finite dimension other than zero.

The beam cluster 4 penetrates a cylindrical examination area 13 in whichthere is located a periodically moving object (not shown). In thisexample of embodiment, said object is a beating heart which carries outintrinsic movements and in some circumstances also moves back and forthon account of the patient's breathing movements. In other embodiments,other periodically moving organs of the body, such as the liver or thebrain, periodically moving parts of organs of the body or periodicallymoving technical objects could also be irradiated.

After passing through the examination area 13, the beam cluster 4 comesinto contact with a detector unit 16 which is attached to the gantry 1and has a detector surface comprising a large number of detectorelements which in this embodiment are arranged in rows and columns inthe form of a matrix. The detector columns preferably run parallel tothe axis of rotation 14. The detector rows are located in planesperpendicular to the axis of rotation, in this embodiment on an arc of acircle around the radiation source S (focus-centered detector). However,in other embodiments they may also be formed differently, for exampledescribe an arc of a circle around the axis of rotation 14 or berectilinear. Each of the detector elements with which the beam cluster 4comes into contact supplies, in each position of the radiation source, ameasured value for a beam from the beam cluster 4.

The spread angle of the beam cluster 4, designated α_(max), determinesthe diameter of the object cylinder within which the object to beexamined is located during acquisition of the measured values. Thespread angle is defined as the angle enclosed by a beam lying in a planeperpendicular to the axis of rotation 14 at the edge of the beam cluster4 with a plane defined by the radiation source S and the axis ofrotation 14. The examination area 13 or the object or the patient tablemay be moved parallel to the axis of rotation 14 or to the z axis bymeans of a motor 5. However, the gantry may also be moved in thisdirection in an equivalent manner. When a technical object is concernedrather than a patient, the object can be rotated during an examinationwhile the radiation source S and the detector unit 16 remain stationary.

With the aid of the motors 2 and 5, the radiation source S and thedetector unit 16 can describe a trajectory relative to the examinationarea 13, said trajectory running on an imaginary cylindrical surface.This trajectory may, for example, run in a helix-like manner when bothmotors are operating. If, on the other hand, the motor 5 for advance inthe direction of the axis of rotation 14 is idle and the motor 2 ismaking the gantry rotate, the result is a circular trajectory for theradiation source S and the detector unit 16 relative to the examinationarea 13. In this example of embodiment, only the helix-like trajectoryis considered.

During the acquisition of the measured values, the movement of the heartis recorded in a known manner by means of an electrocardiograph 8. Forthis purpose, the chest area of a patient is connected to theelectrocardiograph 8 by way of electrodes (not shown). In otherembodiments, in particular in the case of other moving objects, themovement of the object may be traced in other ways. Thus, for example,the movement information could be obtained from the values measured bythe detector unit 16 themselves, so that there is no need for themovement to be recorded using an additional device, such as anelectrocardiograph. For this, firstly a kymogram is produced from themeasured values, from which icymogram the movement can be derived in aknown manner. A detailed description of this method can be found in“Kymogram detection and kymogramn-correlated image reconstruction fromsubsecond spiral computed tomography scans of the heart,” M. Kachelrieβ,D. A. Sennst, W. Maxlmoser, W. A. Kalender, Medical Physics 29(7):1489-1503, 2002, to which reference is hereby made.

In this example of embodiment, it is assumed that the patient is notbreathing during the measurement. The breathing movement can thus bedisregarded. In other embodiments, the breathing movement could bemeasured, for example, using a deformable chest strap which is connectedto a breathing movement measurement device.

The measured values acquired by the detector unit 16 are fed to areconstruction and image processing computer 10 which is connected tothe detector unit 16, for example via contactless data communication(not shown). Moreover, the electrocardiogram is transferred from theelectrocardiograph 8 to the reconstruction and image processing computer10. The reconstruction and image processing computer 10 reconstructs theabsorption distribution in the examination area 13 and displays it on amonitor 11 for example. The two motors 2 and 5, the reconstruction andimage processing computer 10, the radiation source S, theelectrocardiograph 8 and the transfer of the measured values from thedetector unit 16 to the reconstruction and image processing computer 10are controlled by the control unit 7. The control unit 7 furthermorecontrols the transfer of the electrocardiogram from theelectrocardiograph 8 to the reconstruction and image processing computer10.

In other embodiments, the acquired measured values and the measuredelectrocardiograms can be fed for reconstruction firstly to one or morereconstruction computers which forward the reconstructed data e.g. via afiberoptic cable to the image processing computer.

FIG. 2 shows the sequence of a measurement and reconstruction methodwhich can be carried out using the computer tomography scanner shown inFIG. 1.

After initialization in step 101, the gantry rotates at an angularvelocity which in this example of embodiment is constant. However, itmay also vary, for example as a function of time or of the radiationsource position.

In step 103, the examination area or the object or the patient table ismoved parallel to the axis of rotation and the radiation of theradiation source S is switched on so that the detector unit 16 candetect the radiation from a large number of angular positions. At thesame time as or before the switching on of the radiation source S, theelectrocardiograph 8 is activated so that an electrocardiogram ismeasured at the same time.

Thereafter, the spatial area in which the heart is located within theexamination area is determined. For this, the absorption distribution inthe examination area is firstly reconstructed from the measured valuesat a low resolution without taking the electrocardiogram into account.There is a low resolution for example when a volume of 20×20×20 cm³ isrepresented by 64³ voxels.

In step 105, the measured values are parallel rebinned for the purposesof reconstruction. By means of the parallel rebinning, the measuredvalues are resorted and reinterpolated as though they had been measuredusing a different radiation source (an expanded radiation source whichis arranged on part of a helix and can emit beam fans that are in eachcase parallel to one another) and using a different detector (a flat,rectangular “virtual detector” including the axis of rotation 14).

This is explained in more detail with reference to FIG. 3. Here, 17designates the helix-like trajectory from which the radiation sourceirradiates the examination area. A fan-shaped beam cluster 43, the beamsof which run in a plane that includes the axis of rotation 14, isemitted from the radiation source position S₀. It is possible toconsider that the cone-shaped beam cluster emitted from the radiationsource at position S₀ is composed of a large number of planar beam fanswhich are located in planes parallel to the axis of rotation 14 andintersect at the radiation source position S₀. FIG. 3 shows just one ofthese beam fans, namely the beam fan 43.

Moreover, further beam fans 41, 42, and 44, 45 are shown in FIG. 3, saidbeam fans being parallel to the beam fan 43 and lying in planes that areparallel to one another and to the axis of rotation 14. The associatedradiation source positions S⁻², S⁻¹ and S₁, S₂ are taken up by theradiation source S respectively before and after it has reached theradiation source position S₀.

The beam fans 41 to 45 form a group and define a beam cluster 70 with atent-like shape. A group of beam fans is called a projection. For eachprojection there is then defined a rectangular, virtual detector 60which lies in a plane that includes the axis of rotation 14 and isoriented perpendicular to the parallel beam fans of a projection. Thecorner points of the virtual detector 60 are the penetration points ofthe beams which from the outer radiation source positions come intocontact with the opposite helix section, through this plane. For thebeam cluster 70 in FIG. 3, S 2 and S2 are the outer radiation sourcepositions. Detector elements that are arranged in a Cartesian manner aredefined on the rectangular detector 60, that is to say rows and columnson which the measured values are reinterpolated.

Subsequently, in step 107, the measured values allocated to theindividual beams are multiplied by a weighting factor which correspondsto the cosine of the cone angle of the respective beam. The cone angleof a beam is the angle encompassed by this beam with a plane orientedperpendicular to the axis of rotation 14. If said angle is small, thenthe cosine of the angle is essentially equal to 1, so that step 107 maybe omitted.

In step 109, a one-dimensional filtering with a transmission factor thatincreases in a ramp-like manner with the spatial frequency is applied tothe measured values.

For this, use is made of respectively successive values in the directionperpendicular to the axis of rotation 14, that is to say along a row ofthe virtual detector 60. This filtering is carried out along each row ofthe virtual detector for all groups of beam fans.

In other embodiments the parallel rebinning could be omitted. It is thenknown to modify the filtering since the detector unit is curved forexample in the manner of an arc around the radiation source or aroundthe axis of rotation.

In step 111, the filtered measured values are then used forreconstruction of the absorption distribution in the examination area bymeans of a back-projection. The individual steps of the back-projectionare shown in FIG. 4.

In step 201 a voxel V(x) is defined within a predefinable field of view(FOV). Since the reconstruction is to take place with a low resolution,the amount of voxels may be for example 64³ and the FOV may be 20×20×20cm³. In step 203, a projection, that is to say a group of beam fans, isthen selected which has not yet been used to reconstruct the voxel V(x).If no beam of the projection runs centrally through the voxel V(x), thenthe point at which a central beam would have come into contact with thedetector surface is determined. The associated measured value is thencalculated by interpolating the measured values of adjacent beams. Themeasured value which can be allocated to the beam of the projection thatpasses the voxel, or the corresponding measured value obtained byinterpolation, is accumulated on the voxel V(x) in step 205. In step 207a check is made as to whether all projections have been considered. Ifthis is not the case, then the flowchart branches to step 203. Otherwisea check is made in step 209 as to whether all voxels V(x) in the FOVhave been passed through. If this is not the case, then the methodcontinues with step 201. If, on the other hand, all voxels V(x) in theFOV have been passed through, then the absorption in the entire FOV isdetermined and this reconstruction method is terminated.

In order to determine within the examination area the spatial area inwhich the heart is located, in the following step 113 the heart issegmented in the reconstructed three-dimensional data record.

A simple possibility for the segmentation is manual segmentation. Auser, for example a physician, places markings on the surface of theheart in the three-dimensional data record. These markings are connectedby lines and thus form a network that represents the surface of theheart.

A further possibility for the segmentation consists in the use of adeformable model of the heart which is moved, rotated and scaled in thedata record such that the correlation between the data record and themodel of the heart is maximized. This known segmentation method isexplained for example in “Deformable models in medical image analysis: Asurvey”, Medical Image Analysis, 1(2): 91-108, 1996, to which referenceis hereby made.

Another known segmentation possibility is based on a region growing orregion expansion process, in which a user predefines a so-called seedvoxel in an object that is to be segmented, that is to say in this casein the heart. Neighboring voxels of the seed voxel are then examinedusing an association criterion to determine whether they do or do notbelong to the heart. This association criterion may be for example thefact of whether or not they are included within a range of values of thedata values of the voxel. If a data value lies within the range ofvalues then the corresponding voxel is assigned to the heart. In thenext step, the neighboring voxels of the voxel which has been newlyassigned to the heart are examined with respect to the associationcriterion and in some circumstances also assigned to the heart. Thismethod is repeated until no more neighboring voxels can be assigned tothe heart.

The segmented heart shows the spatial area taken up by the heart in theexamination area.

This determination of the spatial area taken up by the periodicallymoving object in the examination area represents only one embodiment.Any method which makes it possible to determine this spatial area may beused according to the invention. For instance, the examination areacould also be reconstructed using other known reconstruction techniques,such as the n-PI method which is described in “The n-PI Method forHelical Cone-Beam CT”, R. Proska, Th. Köhler, M. Grass, J. Timmer, IEEETransactions on Medical Imaging, Vol. 19, 848-863, September 2000. Thereconstruction could also be carried out using the method from “AGeneral Cone-Beam Reconstruction Algorithm”, G. Wang, T. H. Lin, P. C.Cheng, D. M. Shinozaki, IEEE Transactions on Medical Imaging, Vol. 12,486-496, March 1993. The use of the reconstruction method known as ASSR(Advanced Single-Slice Rebinning) would also be possible, which althoughleading to poorer-quality images compared to the abovementionedreconstruction does require less computational complexity. This methodis published for example in “Advanced Single-Slice Rebinning inCone-Beam Spiral CT”, M. Kachelrieβ, S. Schaller, W. A. Kalender,Medical Physics, Vol. 27, No. 4, 754-772, 2000. A reconstruction methodfor a circular trajectory, along which the radiation source movesrelative to the object, is shown for example in “3D Cone-Beam CTReconstruction for Circular Trajectories”, M. Grass, Th. Köhler, R.Proska, Physics in Medicine and Biology, Vol. 45, No. 2, 329-347, 2000.In reconstruction to determine the spatial area taken up by the movingobject, it is important that the resolution of the reconstructedthree-dimensional data record is selected such that the spatial contentscan be determined for example by means of known segmentation methods.This condition is met, for example, when a volume of 20×20×20 cm³ isrepresented by 64³ or more voxels. Any method capable of segmenting anobject in a three-dimensional data record can be used according to theinvention for the segmentation.

Once the heart has been segmented in the three-dimensional data record,the geometric center of the heart can be determined in step 115 bysimple geometric considerations. Only the geometric center of a knownthree-dimensional object is to be determined. The geometric center wouldbe, for example, the center of gravity of this object if a spatiallyconstant density of the object were to be assumed.

In step 117, a point in time is allocated to each projection determinedin step 105. For this, it is determined which beam fan of a projectionirradiates the geometric center of the heart. The point in time at whichthe measured values corresponding to this beam fan were acquired isallocated to the respective projection.

If in other embodiments no parallel rebinning according to step 105 hasbeen carried out during the determination of the spatial area taken upby the periodically moving object in the examination area, then saidparallel rebinning must be carried out prior to step 117.

For the reconstruction of the periodically moving object while talkinginto account the movement recorded during the measurement, repeatingtime ranges are selected in step 119, where the object has assumed atleast a substantially identical object state in each time range. The atleast one substantially identical object state thus occurs in each timerange. Time ranges in which the object states assumed by the objectduring these time ranges differ from one another as little as possibleare preferably selected. For example, two object states differ littlefrom one another when the difference between two measured signals thatcharacterize the respective object states, that is to say for example inthe case of the heart the difference in the signals from theelectrocardiograph, is small compared to the difference between themaximum and minimum value of the measured signal detected during ameasurement. A difference between two measured signals is small, forexample, if it is less than 1%, 2% or 5% of the difference between themaximum and minimum value of the measured signal detected during ameasurement. In the reconstruction explained further below, use is onlymade of measured values which were acquired during these time ranges. Inthe case of the heart it is useful to select a time range of thediastolic phase of the movement of the heart, since in said phase themovement of the heart is considerably smaller than in the systolicphase. This is shown in FIG. 5. The period of the electrocardiogram Hconsists of a range H₁ with relatively little movement and of a range H₂with a lot of movement. In this example of embodiment the time range H₁is selected for the further reconstruction since in this range theobject states differ less from one another than in the range H₂.

In step 121, those projections which were parallel rebinned in step 105and which were acquired during the time ranges H₁ are determined. Thisis again shown in FIG. 5. In step 117, points in time t₀ and t₁ wereallocated to the projections P₀ and P₁. The points in time t₀ and t₁ liein the time range H₁, so that the projections P₀ and P₁ are used for thesubsequent reconstruction. By contrast, projections to which the pointsin time t₂ and t₃ were allocated are not taken into account since thepoints in time t₂ and t₃ lie in the time range H₂.

If in other embodiments the measured values have not been multipliedaccording to step 107 by the cosine of the cone angle of the beamcorresponding to the respective measured value during the determinationof the spatial area taken up by the object in the examination area, thenthis can be carried out prior to the subsequent back-projection. Thesame applies to the filtering according to step 109.

The measured values of the projections determined in step 121 are thenused in step 123 for the reconstruction of the absorption distributionin the heart by means of a back-projection. The individual steps of thisreconstruction are shown in FIG. 6.

For this, in step 301 a voxel V(x) is determined within a predefinablefield of view (FOV). The amount of voxels may be for example 512³ andthe FOV may be 20×20×20 cm³. In step 303, a projection, that is to say agroup of beam fans, is then selected from the projections determined instep 121, which projection has not yet been used to reconstruct thevoxel V(x). If no beam of the projection runs centrally through thevoxel V(x), then the point at which a central beam would have come intocontact with the detector surface is determined. The associated measuredvalue is then calculated by interpolating the measured values ofadjacent beams. The measured value which can be allocated to the beam ofthe projection that passes the voxel, or the corresponding measuredvalue obtained by interpolation, is multiplied in step 305 by aweighting factor w_(p)(x) . The subscript index in this case denotes theprojection selected in step 303.

The following consideration is used to determine the weighting factorw_(p)(x). During the acquisition, the voxel V(x) is irradiated for thefirst time by the cone-shaped beam cluster at a specific point in time.This point in time is referred to as the sunrise SR. The beam leaves thebeam cluster again at a second point in time. This point in time isreferred to as the sunset SS. A factor α_(p)(x) is then allocated to theprojection selected in step 303. This factor is smaller for projectionswhose point in time allocated in step 117 lies relatively close to SR orSS than for projections whose corresponding point in time does not lierelatively close to SR or SS. Relatively close may mean for example thatthe point in time of the projection is located in a range that adjoinsSR or SS, which corresponds to 5%, 10%, 15% or 20% of the overall rangebetween SR and SS. One exemplary profile of the factor α_(p)(x) as afunction of the position of the point in time of the projection betweensunrise and sunset is shown in FIG. 7. In said figure, the time rangebetween SR and SS is divided into three sections. In the first sectionT₁, which takes up 10% of the time range between SR and SS, the factorα_(p)(x) increases from SR up to a value α_(c). In the following rangeT₂, the factor is constantly equal to α_(c) and in the third range T₃,which also takes up 10% of the time range between SR and SS, the factordecreases again toward SS to zero.

It is furthermore assumed that the movement of the heart is lesspronounced at a point in time which lies in the center of the time rangeH₁, which comprises little movement, than at a point in time lyingcloser to the boundary with the time range H₂, which comprises a lot ofmovement. Therefore, measured values of projections whose points in timelie in the center of the range H₁ are more highly weighted than measuredvalues of projections whose points in time lie closer to the boundarywith the time range H₂. In order to take this into account, a secondfactor β_(p)(x) is assigned to the measured value. The subscript index pin this case denotes a projection and, since a point in time isallocated to each projection, also a point in time. Depending on theposition of the point in time within the time range H₁, this factor mayhave a profile that ensures the following: the factor is greater formeasured values of projections whose points in time lie in the center ofthe time range H₁ than for measured values of projections whose pointsin time lie closer to the boundary with the time range H₂. An exemplaryprofile is shown in FIG. 8.

Since the same point in time has been allocated in step 117 to allmeasured values whose beams belong to a projection, all measured valuesof a projection have the same weighting factor w_(p)(x) for a voxelV(x), which weighting factor is defined by the following equation:

$\begin{matrix}{{w_{p}(x)} = {\frac{{\alpha_{p}(x)}\mspace{11mu}{\beta_{p}(x)}}{\sum{{\alpha_{i} \cdot (x)}\mspace{11mu}{\beta_{i}(x)}}}.}} & (2)\end{matrix}$Here, Σα_(i)(x)β_(i)(x) denotes a sum of all projections which have beendetermined in step 121 and are not redundant.

In step 307, the weighted measured value is accumulated on the voxelV(x). In step 309 a check is made as to whether all projections havebeen considered. If this is not the case, then the flowchart branches tostep 303. Otherwise a check is made in step 311 as to whether all voxelsV(x) in the FOV have been passed through. If this is not the case, thenthe method continues with step 301. If, on the other hand, all voxelsV(x) in the FOV have been passed through, then the absorption in theentire FOV is determined and the reconstruction method is terminated(step 313).

In other embodiments, step 123 and steps 301 to 313 may be replaced byother known reconstruction methods which generate a three-dimensionaldata record from the parallel projections determined in step 121.

Furthermore, the reconstruction in steps 301 to 313 may be restricted toprojections which are not redundant. A projection is redundant withrespect to a voxel V(x) if the beam of the projection which runs throughthis voxel V(x) is oriented in the opposite direction with respect to abeam of another projection that has already been used to reconstructthis voxel.

1. A computer tomography method comprising the steps: using a radiationsource to generate a cone-shaped beam cluster which passes through anexamination area and a periodically moving object which is located inthe examination area, producing a relative movement between theradiation source on the one hand and the object located in theexamination area on the other hand, where a trajectory, along which theradiation source moves relative to the object, runs on an imaginarycylindrical surface that surrounds the object, using a detector unit toacquire measured values which depend on the intensity in the beamcluster on the other side of the object, during the relative movement,recording the periodic movement of the object during the acquisition,reconstructing a spatial distribution of the absorption of theperiodically moving object from the measured values with the aid of therecorded periodic movement of the object, comprising the steps: a)determining the spatial area taken up by the object in the examinationarea, b) subjecting the measured values to parallel rebinning in orderto form a number of groups, where the beams corresponding to themeasured values of each group form beam fans which lie in planes thatare parallel to one another and to the axis of rotation, c) determiningfor each group a measured value whose beam irradiates the spatial areataken up by the object, and allocating to the respective group the pointin time at which this measured value was acquired, d) determining thosegroups whose points in time, allocated in step c), lie within periodic,predefined time ranges, e) reconstructing the absorption distribution inthe object from the measured values belonging to the groups determinedin step d).
 2. A computer tomography method as claimed in claim 1,wherein the determination of the spatial area taken up by the object, instep a), comprises the following steps: reconstructing from the measuredvalues a three-dimensional data record which contains the object, with aresolution which makes it possible to segment the object in thethree-dimensional data record, segmenting the object in thethree-dimensional data record, where the segmented object shows thespatial area taken up by the object in the examination area.
 3. Acomputer tomography method as claimed in claim 1, wherein in step c) thegeometric center of the spatial area taken up by the object in theexamination area is determined and for each group a measured value isdetermined whose beam runs through the geometric center, where the pointin time at which this measured value was acquired is allocated to therespective group.
 4. The computer tomography method of claim 3, whereinthe geometric center is a center of gravity of the spatial area taken upby the object in the examination area.
 5. A computer tomography methodas claimed in claim 1, wherein the periodically moving object is aheart, where the periodic time ranges are predefined with the aid of anelectrocardiograph.
 6. A computer tomography method as claimed in claim1, wherein the object moves less in the periodic, predefined time rangesthan in other time ranges.
 7. A computer tomography method as claimed inclaim 1, wherein the reconstruction is carried out with the aid of afiltered back-projection.
 8. A computer tomography method as claimed inclaim 1, wherein the relative movement between the radiation source onthe one hand and the object located in the examination area on the otherhand comprises a rotation about an axis of rotation and runs in acircular or helix-like manner.
 9. A computer tomography scanner, inparticular for carrying out the method as claimed in claim 1,comprising: a radiation source for generating a cone-shaped beam clusterwhich passes through an examination area and a periodically movingobject which is located therein, a drive arrangement for rotating theobject located in the examination area and the radiation source relativeto one another about an axis of rotation and moving them relative to oneanother parallel to the axis of rotation, a detector unit for acquiringmeasured values, said detector unit being coupled to the radiationsource, a movement recording device, in particular anelectrocardiograph, for recording the periodic movement of the objectduring the acquisition, at least one reconstruction and image processingcomputer for reconstructing the spatial distribution of the absorptionwithin the examination area from the measured values acquired by thedetector unit, with the aid of the periodic movement of the objectrecorded by the movement recording device, a control unit forcontrolling the radiation source, the drive arrangement, the detectorunit, the movement recording device and the at least one reconstructionand image processing computer in accordance with the following steps:using a radiation source to generate a cone-shaped beam cluster whichpasses through an examination area and a periodically moving objectwhich is located in the examination area, producing a relative movementbetween the radiation source on the one hand and the object located inthe examination area on the other hand, where a trajectory, along whichthe radiation source moves relative to the object, runs on an imaginarycylindrical surface that surrounds the object, using a detector unit toacquire measured values which depend on the intensity in the beamcluster on the other side of the object, during the relative movement,recording the periodic movement of the object during the acquisition,reconstructing a spatial distribution of the absorption of theperiodically moving object from the measured values with the aid of therecorded periodic movement of the object, comprising the steps: a)determining the spatial area taken up by the object in the examinationarea, b) subjecting the measured values to parallel rebinning in orderto form a number of groups, where the beams corresponding to themeasured values of each group form beam fans which lie in planes thatare parallel to one another and to the axis of rotation, c) determiningfor each group a measured value whose beam in-adiates the spatial areataken up by the object, and allocating to the respective group the pointin time at which this measured value was acquired, d) determining thosegroups whose points in time, allocated in step c), lie within periodic,predefined time ranges, e) reconstructing the absorption distribution inthe object from the measured values belonging to the groups determinedin step d).
 10. A computer readable medium encoded with computerexecutable instructions for a control unit for controlling a radiationsource, a drive arrangement, a detector unit, a movement recordingdevice and at least one reconstruction and image processing computer ofa computer tomography scanner, the computer executable instructions,when executed by a processor, cause the processor to perform the acts ofclaim
 1. 11. The computer tomography method of claim 1, wherein in stepc), the measured value for each group corresponds to a beam fan thatirradiates the geometric center of the spatial area taken up by theobject in the examination area.